Impact of a truncated invariant chain on in vitro assembly of class II MHC molecules depends on the affinity of invariant chain for a given alpha beta dimer.

The assembly of major histocompatibility complex (MHC) class II alpha and beta chains occurs in the endoplasmic reticulum (ER) with the involvement of MHC class II-associated invariant chain (Ii). The present study investigated the impact of Ii on the assembly of both I-A haplotype-matched and -mismatched alpha and beta chains using an in vitro translation system. The alpha and beta chains of I-Ab, I-Ad and I-Ak were cotranslated in vitro in different combinations with or without cotranslation of a truncated murine Ii (mIi 1-131). The translated products were sequentially immunoprecipitated, first with conformation-dependent monoclonal antibodies, then with conformation-independent antibodies. The results show: (1), Ii did not associate with free A alpha and free A beta chains; (2), mIi 1-131 significantly augmented the amount of properly assembled A alpha b A beta b, A alpha b A beta d, A alpha b A beta k and A alpha k A beta b dimers, but had little affect on the assembly of A alpha d A beta d, A alpha k A beta k, A alpha d A beta b, A alpha k A beta d and A alpha d A beta k; (3), All A alpha A beta dimers whose assembly could be significantly facilitated by mIi 1-131 could be coimmunoprecipitated along with substantial amounts of mIi 1-131. This finding is consistent with prior observations that the impact of Ii on class II molecule assembly is allele specific. Furthermore, these results suggest that the efficient assembly of alpha and beta chains is primarily determined by the affinity between alpha and beta chains and the the high affinity of mIi for A alpha A beta dimers is required for mIi 1-131 to assist proper A alpha A beta assembly, most probably through a mechanism in which Ii stabilizes properly assembled A alpha A beta dimers or promotes folding of associated alpha and beta chains to help achieve a stable dimer state.